The invention is directed to a method for making an optical fiber having optical properties that systematically vary along its length. This method is particularly useful for making dispersion managed (DM) single-mode optical waveguide fibers.
The potentially high bandwidth of single-mode optical fibers can be realized only if the system design is optimized so that the total dispersion is equal to zero or nearly equal to zero at the operating wavelength. The term "dispersion" refers to pulse broadening and is expressed in ps/nm-km. "Dispersion Product" refers to dispersion times length and is expressed in ps/nm.
When telecommunications networks employ multiple channel communications or wavelength division multiplexing, the system can experience a performance degradation due to four wave mixing. This performance degradation occurs when the signal wavelength is at or near the zero dispersion wavelength of the optical transmission fiber. This has necessitated the exploration of waveguide fiber designs which can minimize signal degradation that results from this non-linear waveguide effect. A dilemma arises in the design of a waveguide fiber to minimize four wave mixing while maintaining characteristics required for systems which have long spacing between regenerators. That is, in order to substantially eliminate four wave mixing, the waveguide fiber should not be operated near its zero of total dispersion, because four wave mixing occurs when waveguide dispersion is low, i.e., less than about 0.5 ps/nm-km. On the other hand, signals having a wavelength away from the zero of total dispersion of the waveguide are degraded because of the presence of the total dispersion.
One strategy that has been proposed to overcome this dilemma is to construct a system using cabled waveguide fiber segments some of which have a positive total dispersion and some of which have a negative total dispersion. If the length weighted average of dispersion for all the cable segments is close to zero, the regenerator spacing can be large. However, the signal essentially never passes through a waveguide length where the local dispersion is close to zero, so that four wave mixing is prevented.
The problem with this strategy is that each link between regenerators must be tailored to give the required length weighted average of dispersion. Maintaining cable dispersion identity from cabling plant through to installation is an undesirable added task and a source of error. Further, the need to provide not only the proper dispersion, but also the proper length of cable having that dispersion, increases the difficulty of manufacture and leads to increased system cost. A further problem arises when one considers the need for replacement cables.
Those problems are overcome by the optical fiber disclosed in U.S. patent application Ser. No. 08/584,868 (Berkey et al.) filed Jan. 11, 1996 now U.S. Pat. No. 5894537, the specification of which is hereby incorporated by reference. In accordance with the teachings of the Berkey et al. application, each individual fiber is made to be a self contained dispersion managed system. A pre-selected, length weighted average of total dispersion, i.e., total dispersion product, is designed into each waveguide fiber. Each waveguide fiber is interchangeable with any other waveguide fiber designed for that system link. Thus, the cabled waveguide fibers all have essentially identical dispersion product characteristics, and there is no need to assign a particular set of cables to a particular part of the system. Power penalty due to four wave mixing is essentially eliminated, or reduced to a pre-selected level, while total link dispersion is held to a pre-selected value, which may be a value substantially equal to zero.
In accordance with the Berkey et al. patent application, the dispersion of a DM fiber varies between a range of positive values and a range of negative values along the waveguide length. The dispersion product, expressed as ps/nm, of a particular length, l, is the product (D ps/nm-km*l km). A positive number of ps/nm will cancel an equal negative number of ps/nm. In general, the dispersion associated with a length l.sub.i may vary from point to point along l.sub.i. That is, the dispersion D.sub.i lies within a pre-determined range of dispersions, but may vary from point to point along l.sub.i. To express the contribution of l.sub.i to the dispersion product, expressed in ps/nm, l.sub.i is made up of segments dl.sub.i over which the associated total dispersion D.sub.i is essentially constant. Then the sum of products dl.sub.i *D.sub.i characterizes the dispersion product contribution of l.sub.i. Note that, in the limit where dl.sub.i approaches zero, the sum of products dl.sub.i *D.sub.i is simply the integral of dl.sub.i *D.sub.i over the length I.sub.i. If the dispersion is essentially constant over sub-length l.sub.i, then the sum of products is simply l.sub.i *D.sub.i.
The dispersion of the overall waveguide fiber length is managed by controlling the dispersion D.sub.i of each segment dl.sub.i, so that the sum of the products D.sub.i *dl.sub.i is equal to a pre-selected value over a wavelength range wherein signals may be multiplexed. For high rate systems having long regenerator spacing, the wavelength range in the low attenuation window from about 1525 nm to 1565 nm may be advantageously chosen. In this case, the sum of the dispersion products for the DM fiber would have to be targeted at zero over that range of wavelengths. The D.sub.i magnitudes (absolute value) are held above 0.5 ps/nm-km to substantially prevent four wave mixing and below about 20 ps/nm-km so that overly large swings in the waveguide fiber parameters are not required.
The length over which a given total dispersion persists is generally greater than about 0.1 km. This lower length limit reduces the power penalty (see FIG. 5), and simplifies the manufacturing process.
The period of a DM single-mode waveguide is defined as a first length having a total dispersion which is within a first range, plus a second length having a dispersion which is in a second range, wherein the first and second ranges are of opposite sign, plus a transition length over which the dispersion makes a transition between the first and second range. To avoid four wave mixing and any associated power penalty over the transition length, it is advantageous to keep the part of the transition length which has an associated total dispersion less than about 0.5 ps/nm-km as short as possible.
If the transition regions between the regions of positive and negative dispersion are too long, the dispersion in the central portions of the transition regions will be near zero for some finite length of fiber. This will result in some power penalty due to four wave mixing. The longer the transition regions are, the higher the power penalty. The transition regions should therefore be sufficiently sharp that the fiber power penalty does not cause the total system power penalty to exceed the allocated power penalty budget. Preferably, the transition regions between adjacent areas of fiber are less than 10 meters, preferably less than 5 meters, and most preferably less than 3 meters in length.
A primary requirement of a process for making DM fibers is that it be able to form short transition regions. Moreover, the process of making the DM fiber should not be one that itself induces an excess loss that is unrelated to four wave mixing. Also, the process should be simple and be sufficiently flexible that it can be implemented with a variety of fiber designs and materials. Thus, the DM fiber must be a unitary fiber that is formed by drawing a draw preform or draw blank that includes sections that will form the fiber sections of different dispersion. Such a unitary fiber does not include splices between separately drawn fiber sections, as each splice would introduce additional loss. Ideally, the total attenuation of the unitary fiber is no greater than the composite of the weighted attenuation of each of the serially disposed sections of which it is formed.